Most Gardenia set bloom in the fall. If you are going to trim I would suggest as soon as possible. What cultivars do you have?

I'm a fan of pruning gardenias as soon as they finish blooming. It is getting close to being too late to prune now. The plant may not have time to put on new growth and set buds if we have an early frost. Plants in the southeast have been under stress with too much dry weather this summer so this might not be the year to take a chance with a late summer haircut..

Faidra, I would suggest that you prune your gardenia bushes right after they have finished blooming or after the blooms have faded. (Some varieties do bloom twice a year, so trimming right after the flowers have faded is ideal). Gardenias would normally set their flowering buds in the Fall for the following year's bloom. The Shasta Daisy can be cut back after they, also have finished blooming and feeding them with a blooming fertilizer is recommended in promoting more blooms through out the Summer.